Flame detector



June 11, 1957 R. s. MARSDEN, JR

FLAME DETECTOR Filed June 13. 1952 INVENTOR. 12J.' Mnden/,Jig

Arron/VHS United States PatentV FLAME DETECTGR Ross S. Marsden, Jr., Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Dela- Ware Application June 13, 1952, Serial No. 293,480

12 Claims. (Cl. 340-228) This invention relates to flame detection devices for use in combustion apparatus. In another aspect it relates to an improved ame detecting element.

In various jet type engines considerable diiculty has been encountered in detecting the presence of ame in particular regions of the engine. If flame is permitted to extend beyond the normal combustion chamber, damage may result from the flame impinging upon elements not capable of withstanding the high temperatures or from chemical reactions of reactive gases present within the ilame. A flame containing reactive gases can erode away very rapidly metallic parts and even some nonmetallic parts of the engine. An intermittent flame bath is especially destructive in this regard. While various arrangements have been proposed in the past for the detection of llame, for the most part, these devices are of the thermocouple or thermostat type and dependent upon the heat generated by the llame to give an indication of the ame. These devices thus indicate temperature rather than llame, and as such are not entirely reliable.

It is toward overcoming the diculties inherent in the prior art ame detection devices that the present invention is primarily concerned. To this end there is provided an improved type of llame detector which is responsive to the presence of llame impinging upon an element disposed in the region of the llame being detected. This improved detector gives a rapid response to Ithe presence of ame but does not produce a signal in the absence of flame, that is, the element is not responsive to temperature changes.

Accordingly, it is the object of the present invention to provide an improved arne detector capable of providing a rapid response to flame impinging thereon.

Another object is to provide a llame detector capable of distinguishing between llame and high temperatures which may be produced by a ame.

A further object is to provide a llame detector of simple, rugged construction which is capable of withstanding prolonged contact With high temperature llame.

Various other objects, advantages and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic View, shown partially in section, of the ame detection apparatus of this invention employed in conjunction with arturbojet aircraft engine;

Figure 2 is a fragmentary view of a portion of a combustion chamber containing a modified flame detection element; and

Figure 3 is a fragmentary view of a portion of a combustion chamber containing a second modified flame detection element.

Referring now to the drawing in detail and to Figure l in particular, a turbojet aircraft engine is represented schematically. This engine comprises a casing or housing of essentially circular cross section having an air intake 11 and a discharge nozzle 12. Proceeding from the inlet end to the outlet end, casing 10 contains an air compressor 13, a plurality of combustion chambers 14 arranged annularly around the engine at equally spaced intervals and a gas turbine 16. Air supplied by compressor 13 is employed to support combustion of a liquid fuel injected into combustion chambers 14 through fuel nozzles 17 which are fed by a fuel line 18. The greatly expanded volume of resulting heated gases is fed through turbine 16 and thence outwardly through discharge nozzle 12. The purpose of turbine 16 is to drive air compressor 13 through a shaft 20 connecting the rotor assembly of turbine 16 with the rotor assembly of air compressor 13. Turbine 16 includes a peripheral set of stationaryvanes 22 and a movable set of blades 23 disposed adjacent thereto. Located centrally within the tail pipe of the engine is a discharge air regulating plug 25 ,which is suspended therein by appropriate structure, not shown.`

For satisfactory operation of such a turbojet engine it is necessary that the fuel-air ratio supplied to combustion chambers 14 be regulated so that flame produced within these chambers is not of such length as to impinge upon vanes 22 or blades 23 of turbine 16. This is important because flame impinging upon these elements will greatly reduce the fatigue strength thereof. Vanes 22 and blades 23 normally are constructed of an alloy steel which is not capable of withstanding prolonged contact with ame. It is, therefore, desirable that means be provided to detect the presence of flame in the region adjacent vanes 22. By positioning a iiame detecting element immediately forward vanes 22, suitable adjustment of the fuel and air supplied to combustion chambers 14 can be made as necessary to prevent llame from impinging upon said vanes.

The flame detecting element provided in accordance with this invention comprises a probe 30 disposed immediately forward vanes 22 in each of the combustion chambers 14. Probe 30 consists of `an electrode which is mounted in the wall 31 of chamber 14 by means of a suitable insulating support 32. Electrical leads 34 are con-A nected lto probes 30 and pass through insulating sheaths 35 mounted in casing 10. Leads 34 are joined and connected through a capacitor 39 to one input terminal of a high gain alternating current amplifier 38, the second input terminal of which is maintained at ground potential. The walls of combustion chamber 14 are maintained at ground potential and a resistor 40 is connected between leads 34 and ground. The output terminals of amplifier 38 are connected lto a suitable current responsive device 41.

It has been discovered in accordance with the present invention that if an electrical conductive probe is disposed in the region of a llame, iiuctuating voltages are generated across an impedance element connected between the probe and a point of reference potential by llame impinging upon the probe. It is known that a llame forms an electrically conductive path which is believed to be established by ions present in the flame. It is thought that the impingment of these ions on the probe element disposed ih the flame serves to generate the above-mentioned fluctuating voltages. The detector of this invention is particularly effective for indicating turbulent ames.

Probe 30' is constructed of a material which is both electrically conductive and capable of withstanding prolonged contact with high temperature flames'without disintegrating. To this end, probe 30 can be constructed of a non-corrosive metal such as stainless steel, for example. For the detection of extremely high temperature flames, however, it is preferable to employ a `ceramic such as silicon carbide or a ceramic coated metal such as siliconized molybdenum. Insulating support 32 can be constructed of any material capable of withstanding the high temperatures encountered in combustion chambers. Suitable materials for this purpose include aluminum oxide, magnesium oxide and porcelain. Although silicon car- Patented June 11, 1957 bide vnormally is considered to be an electrical insulator, at elevated temperatures the .electrical resistivity of silicon carbide is lowered considerably, such that it can be considered electrically conductive. The following table illustrates the electrical resistivity of selected materials at 1000 C.:

Electrical resistivity Material: (ohms/cm. cubed) Carborundum brick (silicon carbide) 4.1 Baiixite brick (aluminum oXide)- 17,200 Chemical porcelain 20,000 Periclose (magnesium oxide) Infinite The labove values were obtained from- Refractoriesff F. H. Norton, McGraw-Hill Book Company, Inc., New York, third edition, 1949, page 556` .The foregoing listofi'nateriais, however, should be considered as being illustrativeonly, because the present invention is not limitedA to anyparticular probe or insulator material.; Any substance whichis an electrical conductor at the temperatures,encountered can be employed for the probe element, and any material which is an electrical insulator can be employed to vinsulate the probe from .the Wallsof thecombustion chamber.

VIn Figure 2 there is shown a modification of the probe element in which a probe- 40 is illustrated as extending between the opposite walls of combustion chamber 14 and being mounted on both of said walls by respective insulating supports 32 and 42.

A third modication of the probe element is illustrated inl Figure 3 in which a probe 46 is formed substantially ush with the inner surface of wall 3l of combustion chamber 14. 1lfhe three illustrated embodiments Show that the probe element cantake vvarious forms, the only requirement being that a portion of the probe be disposed in the region lof the ame being detected.

Forpurposes of illustrationthe present invention has been described in conjunction with a turbojet aircraft engine. The exact details `of this engine are, of course, not essential to anunderstanding of the invention, and it is not intended to limit the nventionto any particular form of combustion chamber. The probe an'd associated circuitry can be employed as a flame detector completely independent of any particular combustion chamber. The essence ofthe invention comprises a probe and means for detectingtluctuating voltages generated between the probe and a point of referencepotential by ame impinging uponthe probe. In the illustrated arrangement, this point Aofreference potential conveniently is the grounded wall of the combustion chamber. The output electrical signal from amplier 38, in addition providing an indication of flame, can actuate various control systems or warningdevices as is well understood by those skilled in the art.

While the present invention has been described in conjunction with present preferred embodiment thereof, it is to be understood that this description has been illustrative only and not as a limitation of the invention.

What is claimed is:

1. Flame detecting apparatus comprising, in combination, an electrode adapted to be positioned in the region of llame, an alternating current amplifier,v means connecting the respective input terminals of said amplifier to said electrode and tb a point of reference potential, said means including filter means to block the passage of unidirectinoal current flow, and means connected to the output terminals of said amplifier to detect the output `signal of said amplifier, which signal is a function of the fluctuating potential developed between said electrode and said point of reference potential solely by llame impinging upon said electrode.

2. Apparatus for detecting anie in a combustion chamber comprising, in combintion, an electrode positioned in said chamber and electrically insulated therefrom, an alternating current amplifier, means connecting the respective input terminals of said amplifier to said electrode and to a point of reference potential, said means including filter means to block thepassage of unidirectional current ow, and means connectedto the output terminals of said amplifier to detect the output signal of said amplifier, which signal is a `function of the uctuating potential developed between saidelectrode and said point of reference potential solely by ame impinging upon said electrode.

3. The combination in accordance with claim 2 wherein said electrode is secured to but electrically insulated from a wall of said combustion chamber, said electrode being substantially ush with the interior surface of the wall of the combustion chamber to which said electrode is secured.

4. The combination in accordance with claim 2 wherein said electrode comprises a bar of electrical vconductive material secured to but insulatedfrorn a wall of said combustion chamber, said bar extending into the interior of said combustion chamber.

5. The combination in accordance with clai1n4 wherein sa'idbar extends substantially across said combustion chamber.

6. Apparatus for detecting llame in a combustion chamber comprising, in combintion`, an 4electrode pos`itioned in said chamber, mounting support of electrical insulating materialsecuring said electrode to auw/allot said combustion chamber, an alternating current amplitier, means connecting the respective input terminals of said amplifier to said electrode and to a point of reference potential, said means including filter means to block the passage of unidirectional current How, and means con nected to the output terminalsof said amplifier to detect theoutput signal of said amplifier, -whi'ch signal is a 'function of theiiuctuating potential developed between said electrodeand said point of reference potential solely by flame impiuging upon said electrode.

7. The combination in accordance with 'claim 6 wherein said electrode is constructedof stainless steel.

8. The combination in accordance vwith claim 6 wherein s'a'ideletrode is constructed of silicon carbide.

9. The combination in accordance with claim 6 wherein said electrode is constructed of siliconiied molybdenum.

10. The combinationin accordance with` claim 6 wherein said mounting support is' constructed of aluminum oxide.

11. vThe combination in accordance with claim 6 wherein said mounting support is constructed `of magnesium oxide.

1 2.` IThe combination in accordance with claim 6 wherein said mounting support is constructed of porcelain,

References Cited in theile of this patent UNtTED STATES PATENTS 2,003,624 Bower June 4, 1935 2,297,821 Whempner O ct. 6, 1 942 2,313,943 Jones Mar. 16, 1943 2,410,524 Richardson et al. Nov. 5, 1946 2,455,351 Beam et al. D ec. 7, 1,948 2,478,373 Dahline Aug. 9, 1949 2,514,918 Wannamker July 11, 1950 2,517,976 Clarke Aug. 8, 1950 2,523,017 Harrison Sept. 19, 1950 2,538,642 Gardiner et al. Ian. 16, 1951 2,546,415 Alcock Mar. 27, 1951 2,715,815 Malick let al. Aug. 23, 1955 2,742,756 De Boisblanc Apr. 24, 1956 

